The invention relates to a joint structure for power cables with an improved intermediate or termination joint section of the condenser type.
A joint structure of power cables has been known for many years. In this structure, an insulating cover layer is formed around the cable conductor and around this insulating cover layer a reinforcement insulating layer of the condenser type is formed by winding thereon insulating tape and conductive tape alternately.
With this structure, however, the insulating tape and conductive tape must be wound by hand, which involves a great deal of labor. Moreover, wrinkles and shiftings tend to occur in the conductive tape, resulting in dielectric breakdown even at low voltage. Another disadvantage is that the electric field is concentrated at the edges of the conductive tape, leading to severe stress in the insulating layer. Examples of joint structures in which such drawbacks have been corrected are Japan Patent Showa 46-39,037 and U.S. Pat. No. 3,538,241.
Japanese Patent Publication 46-39,037 discloses an embodiment of the invention concerning the termination joint section. In this joint structure, a flexible insulating braid is wound in a specific shape around the insulating cover in termination of a cable. Conductive paint is applied on the insulating braid made of rubber or plastic at specific intervals, so that electrodes are formed consisting of conductive layers of a specific width. An insulating cover is formed on the outside of the braid.
In this prior art, however, in order to mitigate the large voltage stress in the vicinity of a cable shielding layer, the intervals of said electrodes are narrowed and hence the electrostatic capacities are increased. In order to reduce the bearing of the voltage among the electrodes, it is necessary to increase the number of said electrodes and also to narrow the intervals of these electrodes. Therefore, both the said braid of rubber or plastic and the insulating cover must be thinned. Producing such thin insulating braids is impractical. Also, it is difficult to wind the braid around the insulating cover in termination of a cable. The said electrodes formed by applying a conductive paint are in cylindrical or planar form, so that there are edges in the respective electrodes which cause steep electric fields in the vicinity. Furthermore, in order to eliminate the voids produced between the braids thus wound and thereby improve the voltage-withstand electrical characteristic, the insulating braids after winding are occasionally molded by heating. This heat molding often causes deformation of the said electrodes and flow of the resin forming the outer insulating cover, resulting in a shift of the arrangement of electrodes and hence change in the distribution of electrostatic capacities. Since the electrodes are formed by applying a conductive paint, when the braid is under tension, disadvantages arise, such as the breaking off of the paint film.